At present, in order to remove the nitrogen oxide (NOx) contained in a combustion gas generated by a combustion of a fuel such as a coal or the like within a furnace of the industrial boiler for a power generation or the like, an NOx removal apparatus is provided in a back flow side of an exhaust gas flow path in which a combustion gas (hereinafter, refer to as an exhaust gas) discharged from the furnace is circulated. However, on the other hand, in order to save an operating cost required for a consumption of an ammonia in the NOx removal apparatus, the structure is made such that an amount of NOx generated within the furnace is reduced as little as possible by executing a low NOx combustion in a combustion stage within the furnace.
A method of the low NOx combustion includes a two-stage combustion method of divisionally supplying an air required for the fuel combustion (hereinafter, refer to as a combustion air) in an entire within the furnace, and a method using a low NOx burner having a low NOx function as a burner, and a low NOx combustion using them together is generally executed.
FIG. 20A is a schematic front elevational view showing an example of a structure of a combustion apparatus such as a boiler or the like, and FIG. 20B is a schematic side elevational view of the combustion apparatus. Three stages of burners 2 and one stage of air port (hereinafter, refer to as an after air port (AAP) because the air port exists in a back flow side of a gas flow as seen from the burner) are attached in the furnace defined and formed by a water wall 1 so as to face to each other in four rows. In order to supply the combustion air to each of the burners 2 and the AAP 3, a wind box 4 for burner and a wind box 5 for AAP are respectively placed. The burner 2 executes a combustion in which an air ratio (air amount supplied to the burner/theoretical amount of air) is about 0.8. In other words, the NOx generation can be lowered by executing the combustion in which the air is slightly short in comparison with the air amount (the theoretical air amount theoretically required for a complete combustion of the fuel). However, since a rate of the unburned fuel (hereinafter, refer to as an unburned combustible) is inversely increased, the complete combustion is executed by injecting a shortfall air by the AAP 3 in the back flow side.
As mentioned above, the two-stage combustion method is an effective method for reducing the generating amount of the NOx. In this case, in the low NOx burner, a burner structure is contrived such that a denitration can be executed within a flame formed by the burner, however, a detailed description will be omitted here.
A conventional AAP structure is shown in FIG. 21. A high-temperature combustion air (hereinafter, refer to as a high-temperature air) 8 is supplied to the AAP wind box 5 of the AAP 3 attached to a gas flow downstream side of the burner 2 of the water wall 1 (an upper side of the burner 2), and the high-temperature air is supplied into the high-temperature combustion gas within the furnace so as to form a jet flow. In this case, the combustion air 8 is supplied to the burner and the AAP after a temperature of the combustion air 8 is increased to about 300° C. for improving a power generating efficiency of a plant, generally by maintaining the temperature of the high-temperature combustion gas within the furnace.
A combustion region moves into a downstream side of the furnace at a time of employing the two-stage combustion method. Accordingly, if the mixing of the high-temperature combustion gas within the furnace with the high-temperature air flow from the AAP 3 is bad, the high-temperature combustion gas is discharged from the furnace in a state in which the high-temperature combustion gas and the high-temperature air 8 are not sufficiently mixed. Therefore, a lot of unburned combustible (an unburned carbon in a coal and a carbon monoxide in the combustion gas) are contained in the exhaust gas from the furnace. Accordingly, in the furnace of the commercial boiler which has a great combustion efficiency and has an influence on an economical efficiency, in order to promote the mixing of the air from the AAP3, an AAP having a structure shown in FIG. 22 is employed (refer to patent document 1 (JP-A-59-109714)). In this structure, the mixing with the high-temperature combustion gas is promoted by the high-temperature air flow which is supplied from a swiveling device 6 and is swiveled. At the same time, it is possible to supply the high-temperature air flow to a center portion of the furnace by injecting a straight flow having a flow amount controlled by a damper 7 to a center portion of the swirling flow so as to secure a spray penetration of the jet flow.
FIG. 23 is a schematic view of an outline structure of a combustion apparatus, for example, disclosed in patent document 2 (JP-A-3-286906) and patent document 3 (JP-U-1-101011). A burner 2, a lower stage port 11 and an upper stage port 12 are placed in a water wall 1. In other words, the AAP is provided so as to be separated into two upper and lower stages. An exhaust gas or a low-temperature air 10 is supplied from the lower stage port 11, and the high-temperature air 8 is supplied from the upper stage port 12.
The burner 2 and the upper stage port 12 realize a normal two-stage combustion method. In this case, a high-temperature portion is formed in an upper portion of the burner within the furnace, and a gas temperature becomes too high by supplying the high-temperature air 8 and NOx tends to be generated. Accordingly, in order to temporarily lower the temperature of the gas within the furnace, the exhaust gas or the low-temperature air 10 is supplied from the lower stage port 11, and the NOx is prevented from being generated.
However, in this combustion apparatus, it is necessary to supply a lot of exhaust gas or low-temperature air 10 for lowering the temperature of the high-temperature combustion gas in the upper portion of the burner within the furnace. Accordingly, a power generation efficiency of the plant is significantly reduced.
FIG. 24 is a schematic view of an outline structure of a combustion apparatus in accordance with further the other prior art. As shown in the drawing, the burner 2 is arranged so as to face in three stages, and the AAP 3 is arranged so as to face in one stage. In the drawing, reference numeral 22 denotes an environmental apparatus such as an NOx removal apparatus or the like, reference numeral 23 denotes an opening and closing valve, reference numeral 24 denotes an air preheater, reference numeral 25 denotes a forced blower (FDF), reference numeral 26 denotes a coal pulverizing machine, reference numeral 27 denotes a chimney, reference numeral 28 denotes an exhaust gas recirculation blower (GRF), reference numeral 41 denotes a furnace, reference numeral 43 denotes a combustion air flow path, reference numeral 70 denotes an exhaust gas, reference numerals 71, 72 and 73 denote a heat exchanger tube, and reference numeral 74 denotes a furnace bottom gas supply chamber for supplying the exhaust gas to a bottom portion of the furnace.
A distribution within the furnace of an NOx concentration in the combustion apparatus having the structure is shown in FIG. 25. A horizontal axis in the drawing shows the NOx concentration, and a vertical axis shows a distance in a furnace height direction.
In the case that the flow amount of the air supplied from the burner in accordance with the two-state combustion is less than a theoretical air flow amount as shown in the drawing, the gas within the furnace until the air for the two-stage combustion is mixed is constituted by a reducing atmosphere, and the NOx generated in the burner region is gradually lowered. Since the atmosphere is changed to an oxidizing atmosphere by supplying the air for the two-stage combustion by the AAP, the amount of NOx is increased as shown by a solid line in the prior art. The increased NOx is constituted by two kinds NOx caused by the oxidization of the unburned nitrogen compound contained in the combustion gas, and caused by the oxidization of the nitrogen in the air under the high temperature (thermal NOx). In the pulverized coal firing, an NOx level is widely lowered on the basis of a high development of a low NOx combustion technique.
In conventional, a subject in which the NOx is reduced is mainly constituted by a fuel NOx originated from the nitrogen in the fuel, however, in recent days when the NOx level can be set to be equal to or less than 200 ppm, an existence of the thermal NOx is unable to disregard. As a result of a combustion simulation, it has been known that the thermal NOx comes to about one half of an entire NOx generation amount. Further, it has been known that most of the thermal NOx is generated after supplying the air for combustion (which may be called as an air for two-stage combustion) from the AAP. Further, it has been known that the unburned combustible existing in the high temperature portion in the upper portion of the burner becomes a high temperature locally in the early stage of the combustion caused by the air for two-stage combustion, and the thermal NOx is suddenly generated.
A description will be given in detail of the phenomenon with reference to FIG. 26. This drawing shows an AAP structure in accordance with the prior art provided in the water wall 1, and a mixed state of an injected air from the AAP and a high-temperature combustion gas within the furnace 41, and the AAP structure is of a type having two flow paths in the case of this example.
The air for two-stage combustion (an AAP primary air 105 and an AAP secondary air 106) is injected into the furnace 41 through an AAP primary air flow path 102 in a center side from a two-stage combustion air wind box 101, and an AAP secondary air flow path 103 in an outer peripheral side. A proper swivel is applied to the AAP secondary air 106 by an AAP secondary air register 104. In this case, reference numeral 1000 denotes an opening portion for introducing as an AAP primary electricity into the AAP primary air flow path 102 from the two-stage combustion air wind box 101.
In the sight of the combustion promotion for improving the power generating efficiency of the plant, the high-temperature air is frequently used for the air for two-stage combustion. In order to reduce the unburned combustible, it is necessary to promote the mixing of the air supplied from the AAP and the high-temperature combustion gas within the furnace. In order to promote the mixing, since it is necessary to make the air jet flow to reach the center portion of the furnace and to widen a width of the jet flow so as to prevent a gap from being generated between the jet flows, there are executed increasing a spray speed of the air jet flow so as to strengthen the spray penetration of the jet flow, applying the swivel to the air jet flow and the like. In each of the cases, an intensity of turbulence becomes large in the mixed region between the air and the high-temperature combustion gas. When the intensity of turbulence becomes large, the oxidation reaction in the mixed region is promoted, and a local temperature is increased. Further, since a sufficient air is supplied to the mixed region, an oxygen concentration is in a high state. Accordingly, in the mixed region, there is established a condition of high temperature and high oxygen concentration corresponding to a requirement for generating the thermal NOx.
As a technique of lowering the thermal NOx, an exhaust gas mixing of mixing a part of the exhaust gas with the combustion air is frequently used in an oil firing boiler and a gas firing boiler. FIG. 27 shows an outline structure of a combustion apparatus to which the exhaust gas mixing is applied.
A part of the exhaust gas is returned by the gas recirculation blower 28, and a part thereof is supplied into the furnace from the furnace bottom gas supply chamber 74, and is used for controlling a temperature of a reheat steam. Further, a part of the exhaust gas is branched in an outlet of the gas recirculation blower 28 for lowering the NOx so as to be introduced into the combustion air flow path 43 through the gas mixing flow path 29. Reference numeral 30 denotes a gas mixing regulating damper provided on the gas mixing flow path 29.
The combustion air with which the exhaust gas is mixed is supplied into the furnace from the burner 2 and the AAP 3. The exhaust gas mixing is a method which can effectively lowering the thermal NOx on the basis of the reduction of the combustion temperature and the reduction of the oxygen concentration in the combustion field. This method can be applied to the boiler employing the oil or the gas which has a high combustion speed as a fuel, with no problem. However, when applying the exhaust gas mixing to the coal firing boiler having a comparatively low combustion speed, the combustion efficiency is largely lowered on the basis of the reduction of the combustion temperature in an entire of the combustion field and the reduction of the oxygen concentration.
Further, in the low NOx coal burner flame, there exists an NOx removing reaction within the flame that the temporarily generated NOx is reduced by an intermediate product, however, it is known that the NOx removing reaction within the flame is improved in the NOx removing efficiency in accordance that the temperature of the flame becomes high. When the flame temperature is reduced by the exhaust gas mixing, there is a case that the generated NOx is rather increased on the basis of the reduction of the NOx removing efficiency.
As mentioned above, the two-stage combustion method has the NOx reducing effect as an entire of the furnace, however, the AAP itself has an effect of generating the NOx. The conventional AAP has a disadvantage that in the case of promoting the mixing between the high-temperature combustion gas and the air within the furnace for achieving the complete combustion by reducing the unburned combustible, the NOx generated in the AAP is increased.
Further, when applying the exhaust gas mixing for reducing the thermal NOx of the coal firing combustion apparatus as mentioned above, there is a disadvantage that an adverse effect such as the reduction of the combustion efficiency and the reduction of the NOx removing reaction within the flame is generated.